1. Field of the Invention
The invention relates to X-ray tubes with rotating anodes and, more particularly, to a device for the shielding of the stator of the drive motor of the rotating anode.
An X-ray tube for medical diagnosis is generally constituted (FIG. 1) like a diode, i.e. with a cathode 11 and an anode 12 or anti-cathode, these two electrodes being enclosed in a vacuum-tight casing 14 that provides for the electrical insulation between these two electrodes. The cathode 11 produces a beam 13 of electrons and the anode receives these electrons on a small area that constitutes a focal spot from which the X-rays are emitted.
When the high supply voltage is applied by a generator 15 to the terminals of the cathode 11 and of the anode 12 so that the cathode is at the negative potential -HT, a current known as an electron current is set up in the circuit through the generator 15 producing the high voltage. The electron current goes through the space between the cathode and the anode in the form of the beam 13 of electrons which impinge onto the focal spot.
A small proportion of the energy used to produce the electron beam 13 is converted into X-rays, the rest of this energy being converted into heat. Hence, in view also of the high instantaneous power (in the range of 100 KW) brought into play, manufacturers have long been making X-ray tubes with rotating anodes where the anode is made to rotate in order to distribute the heat flux on a ring called a focal ring, with an area far greater than that of the focal spot. The value of this approach increases concomitantly with the rise in rotational speed (generally between 3,000 and 12,000 rpm).
The standard type of rotating anode has the general shape of a disk with an axis of symmetry 16 about which it is made to rotate by means of an electrical motor 17; the electrical motor has a stator 18 located outside the casing 14 and a rotor i 9 mounted in the casing 14 of the X-ray tube and positioned along the axis of symmetry 16, the rotor being mechanically fixed to the anode by means of a supporting shaft 20.
Owing the high levels of energy dissipation, the X-ray tube gets heated and it has to be cooled by being placed in a chamber, called a housing, in which there flows a cooling and insulating fluid that can be cooled by an appropriate device. This housing, made of a metal lined internally with a layer of lead, is also used to protect the external environment against the X-rays emitted by the focal spot of the X-ray tube in every direction.
The combination of the housing and of the tube then forms what is called an X-ray unit.
In an X-ray unit, the X-ray tube, unlike so-called passive components such as resistors, inductors and capacitors which behave according to established laws, is an active or reactive type of component that generates random disturbances against which protection must be provided.
For, the X-ray tubes that are used in medical X-ray diagnosis are vacuum tubes that work at very high voltages going up to 150 kilovolts. These high voltages prompt very strong electrical fields in the vacuum. These strong electrical fields are intensified by the presence of impurities or micro-aggregates on the surface of the electrodes which it is difficult to eliminate during the manufacture of the tube despite all the care with which the surface treatment operations are carried out. If the intensity of the electrical field becomes high enough, then an instability known as a "tube reaction" or "tube crackling" appears, and vaporises all or a part of the impurity that has caused this high intensity of the electrical field. If the surface, in its new state, is not homogeneous enough to reduce the localized intensity of the electrical field to a lower value, then the "crackling" is repeated until the surface is sufficiently homogeneous or "clean" to stand the high voltage.
This phenomenon appears from time to time throughout the life of the tube, and is the means by which the X-ray tube cleanses itself of the impurities that may shift randomly during the life of the tube.
These electrical discharges in the tube excite the natural resonances of the electrical circuits within the housing and the high frequency oscillations that result therefrom, which are typically in the range of a hundred megahertz, leak out and get radiated throughout the electronic equipment placed in the vicinity of the X-ray tube. These oscillations are often very high power oscillations and may, in that case, cause permanent damage to the sensitive electronic components, thus leading to malfunctioning of the electronic equipment.
2. Description of the Prior Art
The conventional means used to reduce the "tube crackling" effect on the electronic equipment are aimed at preventing high frequency parasites from entering the electronic equipment by enclosing the equipment in metal casings, placing filters at the inputs of the equipment and grounding the different elements of the equipment.
Furthermore, since the X-ray tube and the high-voltage generator are positioned in metal casings, the only elements that are not protected are the supply conductors of the cathode and of the anode, as well as those of the stator.
A known way of protecting the supply conductors of the cathode and of the anode is to use coaxial cables of a special type, comprising an external shielding ground-connected to the metal casing of the housing.
Another known method of in reducing the propagation of the high frequency oscillations for the wire of the stator consists of the series connection of inductors to said supply wires and the parallel connection of capacitors between the latter and the ground. Furthermore, to protect the stator itself, there is a known way of placing metal screens which are positioned, outside the tube, between the rotor and the stator. These metal screens are costly. Their mechanical fastening is difficult for the process of mounting them may cause deterioration to the wires of the stator. Their shapes should be rounded to prevent field effects between the stator and the anode. Their thickness of some tenths of a millimeter causes losses of driving currents and creates a heat screen limiting the heat dissipation of the stator.